Respiratory insufficiency after major thermal injury is a source of significant morbidity and mortality. Some pulmonary dysfunction is found in virtually all patients with burns over 40% of body surface. The pathophysiology of the lung damage remains undefined due in part to the lack of a sensitive method of studying pulmonary microvascular integrity. We use lung lymph flow and lymph protein transport indices of fluid filtration rate and vascular permeability to protein to monitor the microcirculation. The unanesthetized sheep appears to be ideally suited for this study because we can obtain pure lung lymph and because the sheep lung is similar to that of man both morphologically and in its response to trauma. The pulmonary microcirculation can be continuously monitored prior to and for one week after a 50% full thickness body surface burn. Once the pathophysiology is defined, attempts at alteration and prevention can be studied.